It's populated with content from a user's friends and favorite things running the gamut from sports teams to musicians to restaurants.

But the way things are ordered and the reasons why certain pieces of information are displayed more prominently than others aren't always clear to everyday users.

Without revealing all of the magic underneath the stream, Facebook engineers explained some of the ranking algorithms that determine which stories appear at the top of one's feed.

Chris Cox, vice president of product at Facebook, explained that the engineering team "wanted to demystify" News Feed rankings to both the media and the user base of more than one billion people worldwide and counting.

Today, the average user is said to see approximately 1,500 stories per day, based on a random sample of 7,000 daily active users over a one-week period during July 2013.

"For each user, we try to come up with a final score to determine how relevant that particular story is to that particular user," Backstrom summed up. "In the end, those scores build your personalized News Feed."

In describing the path the News Feed has taken since launching in 2006, Cox remarked that that the most interesting thing he noticed about the stream's launch were new "emergent types of activity" -- specifically content deriving from Groups.

Ironically, Cox quipped, the biggest Group was one that detested UI changes on Facebook. But some of the others in the top five were dedicated to global humanitarian causes, notably for breast cancer and Darfur.

But Cox recognized that there is still a lot of mystery about the News Feed "plumbing" and how it works.

The trick, Cox explained, is finding a balance between users don't want to miss anything from friends and content publishers (personal and professional) wanting to get their messages and brands out there.

From Facebook's perspective, it's about enabling as many interactions (Likes, Shares, clicks, comments, etc.) as possible.

Lars Backstrom, the engineering manager of News Feed rankings at Facebook, dived into the scoring system, describing that the model in place for the last few years takes stories since a user's last visit and computes how relevant that is to the viewer.

Higher scoring items are sorted above the fold so that users don't have to scroll as far to find them. Stories are also then sorted in blocks based on each given visit.

"For each user, we try to come up with a final score to determine how relevant that particular story is to that particular user," Backstrom summed up. "In the end, those scores build your personalized News Feed."

The original News Feed ranking system is on the left, while Story Bumping is taking place on the right. Image: Rachel King, ZDNet.

Backstrom continued that the engineering team is "always trying to make Feed as relevant as possible," and that involves a lot of experimentation with algorithms and results.

Thus, whenever the team makes changes, it is only tested on approximately one percent of users.

The team then compares how relevant those results are to the rest of the Facebook user base still set to the default experience.

Backstrom concluded, "Our main goal is to create the best personalized newspaper for all of our users."

Not surprisingly, Facebook is ready to mix up the News Feed once again. At least three big changes are already on the way:

Story Bumping: The biggest change coming soon, according to Backstrom. It debugs some of the scoring rankings in the attempt to ensure users don't miss anything from their friends that might have been missed just below the fold. The model is being shifted to instead of taking all new stories and ranking them, but ranking those that are "new to you." Essentially, the News Feed is taking more account of what users see and might pass over. Using the same random sample and time frame, Story Bumping led to five percent more stories visible from friends, eight percent more stories from Pages, and 57 to 70 percent more stories read overall.

Last Actor: Keeps track of the most recent "actor" you've interacted with in real-time. Backstrom stipulated this one is "less impactful," but still helps keeps things relevant. For example, hitting "Like" on a particular update/story at 8AM published by another user "signals" that you like what that friend is doing in particular that day. Thus, the News Feed will bump up that user's stories in the rankings for the rest of the day.

Chronological by Actor: This one is still being developed and won't launch to all users immediately. This one attempts to better integrate and sort stories based the competing factors of rankings and time published related to a particular user/friend/contact. Backstrom acknowledged that there is "still a little more work to do" before that one is rolled out to the masses.

Going forward, Backstrom stressed that Facebook is trying to be more transparent in changes to the News Feed, which historically haven't always been received well.

"We've done a lot of this work behind the scenes," Backstrom admitted. "We haven't talked much about that externally, and there's been a lot of speculation. We haven't done a great job of communicating changes."

He concluded, "Our main goal is to create the best personalized newspaper for all of our users."

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